1. Field of the Invention
The invention relates to a neutral valve structure applied to an HST or an axial piston device forming the HST.
2. Related Art
In a hydrostatic transmission (HST) including a hydraulic pump body and a hydraulic motor body that are fluidly connected to each other by a pair of operation fluid lines, and configured to non-stepwisely change the rotational speed that is inputted into the hydraulic pump body and output the same through a motor shaft supporting the hydraulic motor body by varying an oil supply/suction amount of at least one of the hydraulic pump body and the hydraulic motor body based on the operation of an output adjusting member. The following two configurations have been conventionally proposed in order to obtain a neutral state (a state where the output of the hydraulic motor unit is zero), without locating the output adjusting member at a neutral point strictly.
In a first configuration, a charge line for supplying pressure oil to the pair of operation fluid lines is constantly connected with at least one (for example, a rearward-movement high-pressure-side operation fluid line becoming high pressure when a vehicle moves rearward) of the pair of the operation fluid lines via an orifice (see, for example, U.S. Pat. No. 6,425,244).
In detail, check valves are respectively inserted between the charge line and the pair of operation fluid lines. In the configuration, by providing the orifice in the check valve, which is, for example, provided between the charge, line and the rearward-movement high-pressure-side operation line, it is possible to prevent hydraulic difference between the pair of operation fluid lines from occurring against an operator's intention.
However, the first configuration is not preferable in view of the transmission efficiency of the HST, since the operation oil is constantly leaked from the operation fluid line on the side at which the orifice is provided.
That is, if a diameter of the orifice is reduced, the transmission efficiency of the HST could be improved. However, on the other hand, a neutral width is reduced.
On the contrary, if the diameter of the orifice is increased, the neutral width could be increased. However, the transmission efficiency of the HST is worsened.
A second configuration in which a neutral valve is inserted into at least one of the pair of the operation fluid lines has been proposed in order to solve the fault of the first configuration (for example, see Japanese Unexamined Patent Publication No. H11-351382).
In detail, the neutral valve includes a valve body inserted into the corresponding operation fluid line in a slidable manner along the axis line direction, and a biasing member for urging the valve body against the hydraulic pressure of the corresponding operation fluid line.
When the pressing force due to the hydraulic pressure of the corresponding operation fluid line is smaller than the urging force of the biasing member, the corresponding operation fluid line is fluidly connected to a drain line, and when the pressing force due to the hydraulic pressure of the corresponding operation fluid line exceeds the urging force of the biasing member, the valve body is pushed so that the corresponding operation fluid line is fluidly disconnected to the drain line.
That is, the second configuration is configured so as to prevent the leak from the corresponding operation fluid line through the drain line to prevent the deterioration of the transmission efficiency of the HST in a case that the hydraulic pressure of the operation fluid line is increased, while fluidly connecting the operation fluid line to the drain line so as to secure the neutral width of the HST in a case that the hydraulic pressure of the corresponding operation fluid line is low.
However, since the biasing member receives the hydraulic pressure of the corresponding operation fluid line as it is in the second configuration, a biasing member having a large spring load has to be used, and thereby there was a problem of causing enlargement and high production costs.
The invention has been accomplished in view of the prior art, and it is an object of the invention to provide a neutral valve structure applied to an HST or an axial piston device constituting the HST, capable of securing the neutral width without causing the deterioration of the transmission efficiency of the HST and capable of attaining the miniaturization and the low cost.